The Global Positioning System (GPS) is a satellite based system developed by the United States Department of Defense to give positional information to a GPS receiver anywhere in the world. A properly equipped GPS receiver may therefore be used to provide positional information when position is desired. The GPS system is enabled by 24 or more satellites orbiting around the Earth at a period about 12 hours and a plurality of ground control stations. The aforementioned satellites make up a constellation and are arranged in six orbiting planes. Now, the orbiting planes are spaced sixty degrees apart and are inclined approximately fifty-five degrees from the equatorial plane. Such a kind of arrangement ensures that at any time and any location on the Earth, neglecting obstacles such as mountains and tall buildings, a GPS receiver could receive signals from 4 to 11 GPS satellites.
The data broadcasted by any GPS satellite is known as navigation message. The navigation message includes a plurality of information, such as ephemeris, almanac and a GPS satellite based time reference. Ephemeris herein refers to a set of data that indicates positions of GPS satellites. Almanac herein refers to a set of data that describes the orbits of the complete active fleet of GPS Satellites. The GPS satellite based time reference is a high accurate time reference generated by an atomic clock on each GPS satellite.
Normally, the navigation message is a narrow band binary phase shift keyed (BPSK) signal with a data rate of 50 bit per second. In order to enhance the performance of the GPS system, the navigation message of each GPS satellite needs to be spreaded over a wide band signal. Thus, the navigation message is first modulated with a high rate repetitive pseudo-random noise (PRN) code. Before transmission, the modulated navigation message needs to be further modulated with a high frequency carrier wave.
In order to determine the 3-dimensional (3D) position of a GPS receiver, the GPS receiver needs to obtain the positions and pseudoranges of at least four GPS satellites. Pseudorange herein refers to the distance between a GPS receiver and a GPS satellite. The position of a GPS satellite can be calculated from the ephemeris. And pseudorange can be calculated from time interval between the time when a GPS satellite transmits a GPS satellite signal and the time when the GPS satellite signal is received by a GPS receiver. Therefore, positions and pseudoranges can be calculated based on navigation messages. In order to obtain the navigation message, a GPS receiver needs to acquire and track GPS satellite signals.
In a GPS satellite signal acquisition phase, a GPS receiver first generates a local carrier and employs the local carrier to demodulate a GPS signal. However, the satellites are orbiting around the Earth at high speed and the GPS receiver may also be moving, therefore the carrier frequency of a GPS satellite signal may shift as a result of Doppler Effect. The carrier frequency shift of a GPS satellite signal caused by Doppler Effect is known as carrier Doppler frequency shift. Conventionally, it is reasonable to assume that the maximum carrier Doppler frequency shift is about ±10 kHz. Thus, the GPS receiver may need to acquire the carrier frequency of a GPS satellite signal in a range of ±10 kHz.
Still in the GPS satellite signal acquisition phase, the GPS receiver may also need to search for the PRN code phase of a GPS satellite signal and eliminate the PRN code phase error. The PRN code phase error herein refers to the code phase difference between the PRN code used by a GPS satellite and the PRN code generated by a GPS receiver. Besides the carrier Doppler frequency shift and PRN code phase error, the reference frequency provided by the local oscillator and the local time reference of a GPS receiver may not be accurate and may contain errors. The GPS receiver also needs to eliminate the reference frequency error and the local time reference error. In a GPS satellite signal tracking phase, all the aforementioned errors may be eliminated and the navigation message is obtained. Theoretically, a GPS receiver needs a minimum of about 18 seconds to calculate the position of the user. However, the signal of each GPS satellite does not reach the GPS receiver at the same time and GPS receiver needs time to acquire each GPS satellite. Therefore, conventionally, it takes 30 seconds to several minutes for a GPS receiver to finish GPS satellite signal acquisition and tracking and calculate the position of the user.
Many parameters are utilized to evaluate the performance of a GPS receiver. One of the parameters is the time delay from the time when a GPS receiver is powered up to the time when the GPS receiver determines current position for the GPS receiver. This parameter is known as the Time To First Fix (TTFF). Generally, GPS receivers with the shortest TTFF are preferred. The TTFF of a GPS receiver is affected by individual hardware and software design of the GPS receiver. As mentioned above, the TTFF of a conventional GPS receiver may range from 30 seconds to several minutes.
In order to reduce the TTFF of a GPS receiver, many solutions have been proposed. In one solution, there is provided a GPS receiver capable of storing backup navigation data in a memory when the GPS receiver is powered off, wherein the backup navigation data can be used to reduce the TTFF of the GPS receiver. Backup navigation data includes ephemeris, almanac and GPS satellite based time reference of each GPS satellite. Backup navigation data may also include other information such as, time mark, and the position of the user etc. Time mark indicates the time when the backup navigation data is generated.
When the GPS receiver is powered up again, the GPS receiver may use the ephemeris from the backup navigation data to determine the position and carrier Doppler frequency shift of any GPS satellite. The GPS receiver may also use the almanac to determine approximate positions of GPS satellites. When the approximate positions of GPS satellites are determined, the GPS receiver is capable of using the approximate positions of GPS satellites to estimate the carrier Doppler frequency shift of each GPS satellite. The GPS receiver may also employ the GPS satellite based time reference to synchronize the local time reference. When a GPS receiver gets the position of a GPS satellite or the carrier Doppler frequency shift of a GPS satellite signal, the GPS receiver does not need to acquire the carrier frequency of the GPS satellite in a wide range. Thus, the TTFF of the GPS receiver could be greatly reduced.
However, under the control of the aforementioned plurality of ground control stations, the ephemeris and the almanac change from time to time. The ephemeris is updated once every a few hours meanwhile the almanac is updated every a few days. Therefore, the GPS receiver may need to verify whether the backup navigation data stored in the memory is valid after powered up. Thus, in the aforementioned solution, there is also provided a real time clock (RTC) circuitry which is powered by a battery and capable of providing a local time reference while the GPS receiver is powered off. When the GPS receiver is powered up again, the local time reference provided by the RTC circuitry can be used to verify the backup navigation data.
FIG. 1 illustrates a prior art block diagram of a GPS receiver. The GPS receiver 106 employs a battery powered RAM to store the backup navigation data and a battery powered RTC circuitry to provide a local time reference. The GPS receiver 106 as illustrated in FIG. 1 is capable of using the backup navigation data and the local time reference to reduce the TTFF during determination of a position for the GPS receiver.
The GPS receiver 106 comprises a positioning unit 100 capable of processing GPS signals and calculating positions for the GPS receiver, a RAM 102 in communication with the positioning unit 100, a RTC circuitry 104 for providing a local time reference to the positioning unit, and a battery 108 for providing power to the RAM 102 and the RTC circuitry 104.
When the GPS receiver 106 is powered on, the positioning unit 100 continues to process incoming GPS signals, obtain the navigation message and the GPS satellite based time reference and calculate positions for the GPS receiver. Since the local time reference of the GPS receiver 106 contains an error, the positioning unit 100 needs to synchronize the local time reference to the GPS satellite based time reference. In order to reduce TTFF, the positioning unit 100 may also generate and store backup navigation data to the RAM 102 at predetermined time intervals. The RAM 102 is capable of storing the backup navigation data and preventing loss of the backup navigation data with a continuous power supply when the GPS receiver 106 is powered off.
When the GPS receiver 106 is powered off or the power delivered to the GPS receiver 106 is accidentally interrupted, the battery 108 provides power to the RAM 102 and RTC circuitry 104. Thus, when the power supply to the GPS receiver is interrupted, the battery 108 ensures that the backup navigation data stored in the RAM 102 is preserved and the RTC circuit 104 keeps operating.
When the GPS receiver 106 is powered up again, the positioning unit 100 reads out the backup navigation data stored in the RAM 102 and obtains the local time reference provided by the RTC circuitry 104. Although the local time reference provided by the RTC circuitry 104 contains an error, the local time reference can be employed to verify the backup navigation data stored in the RAM 102. To verify the backup navigation data, the positioning unit 100 needs to calculate the time interval between the local time reference and the time mark from the backup navigation data. If the time interval exceeds a predetermined period, the backup navigation data is regarded as invalid and the GPS receiver 106 begins to acquire a GPS satellite signal in a wide frequency range. As a result, the acquisition of 4 GPS satellite signals takes a lot time. When the backup navigation data is valid, the positioning unit 100 employs the backup navigation data to determine a position for the GPS receiver after the GPS receiver is powered on and avoids acquiring a GPS satellite signal in a wide frequency range. Consequently, the TTFF of the GPS receiver 106 is reduced.
In the aforementioned solution, in order to reduce the TTFF, both the RTC circuitry and the RAM are necessary and the power supply to the RTC circuitry and the RAM should not be interrupted. However, there are situations when the power of the battery 108 is exhausted or the battery 108 is disconnected either from the RAM 102 or from the RTC circuitry 104. Thus, after the GPS receiver is powered on again, the RTC circuitry 104 cannot provide the local time reference to the GPS receiver 106. Consequently, the backup navigation data cannot be used to determine the position of a GPS receiver for reducing TTFF purpose under the circumstance when battery runs out or removed for a period of time.
Therefore, it is needed a system that is capable of reducing the TTFF of a GPS receiver without using a RTC circuitry and immune to power loss, and it is to such system that the present invention is primarily directed.